Mollusk Carbonate Thermal Behaviour and Its Implications In

1 Mollusk carbonate thermal behaviour and its implications in

2 understanding prehistoric fire events in shell middens

3 Stefania Milano1,2*, Susanne Lindauer3, Amy L. Prendergast4, Evan A. Hill5, Chris O. Hunt6, Graeme 4 Barker7, Bernd R. Schöne2 5 6 7 1 Department of Human Evolution, Max Planck Institute for Evolutionary Anthropology, Deutscher Platz 6, 04103 8 Leipzig, Germany 9 2 Institute of Geosciences, University of Mainz, Joh.-J.-Becherweg 21, 55128 Mainz, Germany 10 3Curt-Engelhorn-Zentrum Archaeometry gGmbH, Klaus-Tschira-Archaeometry-Centre, C4, 8, 68159 Mannheim, 11 Germany. 12 4 School of Geography, University of Melbourne, 221 Bouverie St, Carlton, 3053, VIC, Australia 13 5 School of Natural and Built Environment, Queens University Belfast, Elmwood Ave, Belfast BT9 6AY, UK 14 6 School of Natural Sciences and Psychology, Liverpool John Moores University, Byrom Street, Liverpool, 15 United Kingdom 16 7 McDonald Institute for Archaeological Research, University of Cambridge, Downing St, Cambridge CB2 3ER, 17 UK 18 19 20 21 * Corresponding author. Email: [email protected]

25 Keywords: Carbonate phase transformation; Haua Fteah; Shell microstructure; Raman spectroscopy;

26 Thermal-induced diagenesis; Pyrotechnology

29 Abstract

30 Archaeological shell middens are particularly important for reconstructing prehistoric human

31 subsistence strategies. However, very little is known about shellfish processing, especially when related

32 to the use of fire for dietary and disposal purposes. To shed light on prehistoric food processing

33 techniques, an experimental study was undertaken on modern gastropod shells (Phorcus lineatus). The

34 shells were exposed to high temperatures (200-700 °C) to investigate subsequent mineralogy and macro-

35 and microstructural changes. Afterwards, the three-pronged approach was applied to archaeological

36 shells from Haua Fteah cave, Libya (Phorcus turbinatus) and from shell midden sites in the United Arab

37 Emirates (Anadara uropigimelana and Terebralia palustris) to determine exposure temperatures.

38 Results indicated that shells from the Haua Fteah were exposed to high temperatures (600 - 700 °C)

39 during the Mesolithic period (c. 12.7 - 9 ka), whereas specimens from the Neolithic period (c. 8.5 - 5.4

40 ka) were mainly exposed to lower temperatures (300 - 500 °C). The thermally-induced changes in A.

41 uropigimelana and T. palustris shells from the South East Arabian archaeological sites were similar to

42 those seen in Phorcus spp. suggesting a broad applicability of the experimental results at an interspecific

14 43 level. Although heat significantly altered the appearance and mineralogy of the shells, CAMS ages

44 obtained on burnt shells fit within the expected age ranges for their associated archaeological contexts,

45 indicating that robust radiocarbon ages may still be obtained from burnt shells. Our study indicates that

46 the combination of microstructural and mineralogical observations can provide important information to

47 infer shellfish processing strategies in prehistoric cultures and their change through time.

51 1. Introduction

52 Shells grow incrementally throughout the lifetime of mollusks and function as protection and support

53 structures. Shells also serve as excellent palaeoenvironmental archives (i.e. Jones, 1983; Schöne et al.,

54 2004; Butler et al., 2013), because they faithfully record the physical and chemical conditions of their

55 ambient environment and temporal changes to these. Such information is stored in the form of

56 geochemical and structural properties (Epstein, 1953; Goodwin et al., 2001; Schöne, 2008).

57 Sclerochronology is the research field that studies the temporal context of shell chemical composition

58 (i.e. stable isotopes and trace elements) and physical accretionary patterns to produce extremely highly

59 resolved palaeoenvironmental reconstructions (Schöne et al., 2005; Miyaji et al., 2007; Milano et al.,

18 60 2017; Oschmann, 2009). For example, shell oxygen isotope content (δ Oshell) is routinely used as

61 paleothermometer (Schöne et al., 2005; Ferguson et al., 2011; Prendergast et al., 2013; Prendergast and

62 Schöne, 2017).

64 A rapidly growing interest in the research field of sclerochronology supports the spread of its

65 methodologies and approaches to different disciplines such as archaeology and environmental

66 biomonitoring (Mannino and Thomas, 2002; Andrus, 2011; Steinhardt et al., 2016; Schöne and Krause,

67 2016). The analysis of mollusk shell material is especially relevant within the framework of prehistoric

68 archaeology. Shellfish have been an important dietary component since the emergence of anatomically

69 modern humans (~ 300 kyr ago; de Lumley, 1966), due to their easy accessibility, reliable availability

70 throughout the year and source of proteins and micronutrients essential for physical development

71 (Erlandson, 2001; Broadhurst et al., 2002; Marean et al., 2007; Fa, 2008).

73 The application of sclerochronology substantially broadens the potential use of mollusk shells in

74 archaeology. Besides being suitable palaeoenvironmental archives, the shells become a key tool for

18 75 understanding the response of human behaviour to climatic changes. For instance, δ Oshell is used to

76 constrain the season of mollusk collection (Mannino et al., 2007; Burchell et al., 2013; Prendergast et

77 al., 2016). In turn, seasonality provides information on the seasonal mobility of hunter-gatherer societies

78 and helps to identify whether sites were permanently or ephemerally occupied (Shackleton, 1973;

79 Mannino and Thomas, 2002; Eerkens et al., 2013). However, the processes related to mollusk

80 preparation, consumption and disposal are still largely unknown (Milano et al., 2016). In some cases, it

81 has been observed a modification of the shell structure by removal of the gastropod apex with stone tool,

82 thorn or canine tip to allow the mollusk to be sucked out (Girod, 2011). As for bivalves, the shells were

83 sometimes scarred or wholesale smashed (Hammond, 2014). Evidence of pyrotechnology associated

84 with mollusk shell middens suggests that a certain degree of heat exposure may have been involved in

85 some cases (Erlandson et al., 1999; Berna and Goldberg, 2008; Taylor et al., 2011). However, few

86 studies have addressed the reconstruction of such processes in the framework of prehistoric marine

87 resource exploitation (Andrus and Crowe, 2002; Aldeias et al., 2016; Milano et al., 2016) and this study

88 will be the first instance where this has been carried out.

90 The present study builds upon previous work by Milano et al. (2016) who found that significant

91 structural and chemical changes occurred in modern mollusk shells when they were heated at ≥ 300 °C

92 for 20 minutes or more. Here, we investigate the effects of a shorter heat exposure (5 minutes), since

93 shellfish are generally thought to be processed for short periods of time. To achieve this aim, modern

94 Phorcus lineatus are used as a calibration tool to understand the response to thermal treatments at the

95 macro-, microstructural and geochemical level. These findings are then applied to archaeological

96 specimens from the Haua Fteah, eastern Libya (Phorcus turbinatus) to test whether high temperatures

97 exposure can be detected in archaeological shells of the same genus using the three-pronged approach

98 developed in the experimental phase. Archaeological specimens from the United Arab Emirates

99 (Anadara uropigimelana and Terebralia palustris) are used to understand the efficacy of this three-

100 pronged approach on a broader scale to other mollusk species. Furthermore, given the importance of

101 shells for radiocarbon dating coastal sites and the abundance of burnt shells in middens (Douka et al.,

102 2014; Lindauer et al., 2016), we present preliminary results on the influence of thermal alterations on

103 shell radiocarbon dating results.

104

105 2. Materials and methods

106 A total of 41 shells were analysed (Table 1). Fifteen live-collected shells of P. lineatus were used for the

107 experimental phase. The archaeological material consisted of eighteen specimens of P. turbinatus

108 selected from key occupation contexts in the Mesolithic and Neolithic layers from the Haua Fteah cave

109 in Libya, as well as six specimens of A. uropigimelana and two specimens of T. palustris from shell

110 midden sites near Kalba in the United Arab Emirates (Fig. 1).

111

112 2.1 Shell material: P. lineatus and P. turbinatus

113 Phorcus spp. are rocky shore gastropods living in the intertidal zone along the Mediterranean (P.

114 turbinatus: Menzies et al., 1992; Schembri et al., 2005; Mannino et al. 2008; Prendergast et al., 2013)

115 and Eastern North Atlantic coasts (P. lineatus: Kendall, 1987; Donald et al., 2012; Gutiérrez-Zugasti et

116 al., 2015). Both species are sensitive to seasonal environmental changes, specifically water temperature

117 fluctuations. Furthermore, they are extremely abundant in prehistoric sites of this region (Mannino and

118 Thomas, 2001; Colonese et al., 2011; Hunt et al. 2011; Bosch et al., 2015; Gutiérrez-Zugasti et al.,

119 2015). Therefore, many previous studies have used their geochemical data for palaeoenvironmental

120 reconstructions (Mannino et al., 2003; Colonese et al., 2009; Prendergast et al., 2016).

121

122 In the present study, P. lineatus (previously known as Osilinus lineatus and Monodonta lineata)

123 was selected as a modern reference and P. turbinatus (previously known as Osilinus turbinatus,

124 Monodonta turbinata and Trochocochlea turbinata) for the archaeological case study. Despite being

125 two distinct species, they share similar shell shape, size and the same microstructural organization and

126 mineralogy and they live within the same microenvironments on the Mediterranean and Atlantic coasts.

127 Their shell consists of two aragonitic layers with specific organizations. The outer shell layer (oSL) is

128 arranged in spherulitic prismatic microstructures whereas the inner shell layer (iSL) consists of nacre

129 (Mannino et al., 2008; Milano et al., 2016). The minor phenotypic and taxonomically relevant

130 differences between the two species in shell coloration and aperture size likely do not influence the

131 structural behaviour of the shell in reaction to thermal stress.

132

133

134 2.2 Shell material: A. uropigimelana and T. palustris

135 A. uropigimelana and T. palustris inhabit tropical mudflats. A. uropigimelana is a bivalve of the Arcidae

136 family (ark clams), which is widely distributed in East Africa and the Central Pacific. It lives in shallow

137 waters (1-8 m) in association with seagrass beds and can reach 45 years of age (Tebano and Paulay,

138 2001; Petchey et al., 2013). T. palustris, also known as mud whelk, is one of the largest gastropods of

139 mangrove-dominated habitats in the Pacific Ocean (Plaziat, 1984; Houbrick, 1991; Carlen and Olafsson,

140 2002). It is typically found in the intertidal zone and its lifespan is currently unknown. Possibly the adult

141 size is reached after about four years, when it moves from offshore environments into the mangrove

142 settings (Nishihira et al., 2002). Both species are very abundant in the prehistoric record in the Arabian

143 Sea region (Biagi et al., 1984; Beech and Kallweit, 2001; Gardner, 2005; Lindauer et al., 2017).

144

145 The shell of A. uropigimelana is entirely aragonitic and consists of three layers. The oSL is

146 organized in branching crossed-lamellar structures, the middle shell layer (mSL) is formed of linear-

147 crossed lamellae and the iSL is characterized by irregular complex-crossed lamellae. Likewise, the shell

148 of T. palustris is aragonite and is organized in simple-crossed lamellar (oSL and iSL) and linear-crossed

149 lamellar microstructures (mSL). The terminology follows the author´s observations and the description

150 from Carter et al. (2012).

151

152

153 2.3 Modern and archaeological settings

154 Modern specimens of P.lineatus were collected from Praia do Tamariz, Portugal (38.7036 °N, -9.4038

155 °E) on 10 August 2016. The shells were gathered from a rocky portion in the mid-intertidal zone just

156 outside the estuary of the Tajo River on the Atlantic coast (Fig. 1). To minimize the effects of ontogeny

157 on the stable isotope results, shells with similar sizes were selected. Maximum shell height ranged

158 between 12 and 14.3 mm and the maximum shell width ranged between 13.3. and 15.7 mm. This size

159 class represents the average shell size of P.lineatus adult population in the region (da Costa, 2015).

160 Immediately after collection, the specimens were frozen for about one hour and subsequently the soft

161 tissues were removed.

162

163 The archaeological remains of P. turbinatus were collected from the Haua Fteah cave, Libya (Fig.

164 1). The Haua Fteah cave was first excavated in 1951-1955 by Charles McBurney, who cut a 14 m-deep

165 stepped trench from the level of the present cave floor, exposing a deep sequence of occupation dating

166 from the last intergacial to the Holocene (McBurney, 1967). New excavations (ERC:TRANSNAP)

167 coordinated by Prof. Graeme Barker between 2008-2015 reopened the McBurney trench and cut an

168 overlapping series of small trenches down its southern face (Barker et al., 2010, 2012; Farr et al., 2013).

169 Eleven of the eighteen specimens used in this study were from Trench U, a small (0.75 m x 1 m) cutting

170 in the upper 2 m of the sediments which exposed occupation levels of Neolithic character and age. The

171 Neolithic phase in the cave has a modelled age of ca. 8.5 - 5.4 cal. BP (Douka et al., 2014). These shells

172 were excavated in 2012 from contexts 742, 743 and 747. These sediments, especially context 747, were

173 associated with evidence of extensive burning in form of frequent charcoal, visibly charred animal bone

174 and shell, and lithic artefacts with thermoclastic fractures. Seven specimens were selected from Trench

175 M, a 2 m x 1 m trench cut down the same side of the McBurney trench at ca. 2-8 m depth. The

176 specimens were excavated in 2009 and 2010 from contexts 10,002, 10,005 and 10,006, which have been

177 dated to the Early Holocene and can be ascribed to the Mesolithic Capsian phase of occupation defined

178 by McBurney (1967). The sediments were similar in character to those of Trench U in terms of their

179 plentiful evidence for firing events.

180

181 The archaeological remains of A. uropigimelana and T. palustris were excavated from open sites

182 on the Gulf of Oman coast in the United Arab Emirates (Fig. 1). Specimen KS Ana1, KSM UBL Ana2,

183 KK1 Ana3 and KK1 Ana4 came from the shell midden KK1 situated near the mangroves at Kalba,

184 along the northern coast of the Gulf of Oman (Lindauer et al., 2016). Specimen KS Ana1 was found on

185 the beach nearby and it yielded a radiocarbon age of 7328-7113 cal. BP. It may have remained in the

186 mangrove sediments or in the sabkha (salt flat) since Neolithic times or perhaps washed out recently

187 from an archaeological midden where it was found on the beach. This would also explain its good

188 preservation. The shell midden KK1 is at the outermost edge of the sabkha of the mangrove at Kalba

189 and contains only material from the Neolithic, in two distinct phases. These are reflected in the shell 14C

190 ages that range from ca. 7146 - 6946 cal. BP at the base to ca. 6600 - 6450 cal. BP at the surface (1σ).

191

192 Specimens K4 SL Ana3, K4 SL GA1, K4 BZ GT1 and K4 BZ UGT1 were excavated from site

193 Kalba K4, lying close to the modern town of Kalba north of a mangrove area. This site has evidence of

194 settlement structures that can be ascribed to the Bronze Age and Iron Age. Two distinct layers (MBZ 1

195 and MBZ 2) situated close to an Iron Age mudbrick wall contain a large amount of shells, both burnt

196 and unburnt, together with charcoal fragments. The lower layer (MBZ1) contained fewer shells than the

197 immediately overlying layer MBZ2. Both layers are around 30 cm thick. Dating measurements show

198 that both layers can be ascribed to the Middle Bronze Age (Lindauer et al., 2017). The shells of the

199 lower MBZ1 have a mean age of ca. 3882 - 3727 cal. BP and the upper MBZ2 shells of ca. 3695 - 3641

200 cal. BP. The results are in accordance with the archaeological data from previous excavations (Phillips

201 and Mosseri-Marlio, 2002).

202

203

204 2.4 Experimental design

205 The experiment on modern shells followed a similar protocol to that described by Milano et al. (2016).

206 However, in this case the shells were exposed to different temperatures for 5 min. The choice of this

207 duration was twofold: (1) to identify potential changes in shell behaviour after short (5 min) heat

208 exposure as compared with the medium (20 min) and long (60 min) heat exposures that were tested in

209 the previous study (Milano et al. 2016) and (2) to attempt to reconstruct the temperatures at which

210 archaeological shellfish were processed. Sparse ethnographic observations (e.g. Meehan, 1977, 1982) as

211 well as modern traditions (e.g. Smith, 1978) suggest that the bivalves are processed for only few minutes

212 before consumption.

213

214 The shells were heated at 200 °C, 300 °C, 400 °C, 500 °C, 600 °C and 700 °C in a high

215 temperature tube furnace. Specimens were placed in the furnace when the desired temperature was

216 stabilized to ensure the reproducibility of the experiment and to minimize the noise deriving by

217 temperature fluctuations. In order to record potential weight loss, each shell was weighted before and

218 after the roasting experiment to the nearest 1 mg.

219

220

221 2.5 Shell preparation

222 To study the specimens, modern and archaeological shells were fully embedded in Struers EpoFix resin

223 and air-dried for 24 hours. This strategy prevented the shells from breaking during the following

224 preparation steps. Each resin block was glued to a plexiglass cube and a low-speed precision saw

225 (Buehler Isomet 1000) was used to cut two sections (ca. 2 mm thick) perpendicular to the growth

226 direction. One section of each specimen was glued to a microscope slide with JB KWIK epoxy resin and

227 was later used for geochemical analyses. The other section was used for SEM observations. All shell

228 sections were ground on a Buehler Metaserv 2000 grinder-polisher machine with silicon carbide papers

229 of different grit sizes (P320, P600, P1200, P2500). After each grinding step, the slabs were immersed in

230 de-ionized water and ultrasonically rinsed for 2 minutes. A Buehler VerduTex cloth with a 3 µm

231 diamond suspension was used to polish the sections. The sections used for SEM analysis were etched for

232 5 s in 1 vol% HCl and bleached for 30 min in 6 vol% NaOCl, to remove the organic sheets masking the

233 microstructures.

234

235

236 2.6 Macro-, microstructural and mineralogical analyses

237 Modern shells were photographed before and after the thermal treatments allowing a visual comparison

238 at macroscopic scale. To identify thermal changes, the coloration of the shells was identified according

239 to the Pantone colour system. The sections prepared for the SEM were analysed using a Zeiss Axio

240 Imager.A1m stereomicroscope to record potential changes in the appearance of the two shell layers.

241 Prior to the microstructural analysis, the samples were sputter-coated with a 2 nm-thick layer of

242 platinum using a Leica EM ACE200 Vacuum Coater. The surface of the sections was studied with a 3rd

243 generation LOT Quantum Design Phenom Pro desktop SEM with 10 kV accelerating voltage equipped

244 with a backscatter electron detector. Shell mineralogy was investigated in each shell layer by using a

245 Horiba Jobin Yvon LabRam800 spectrometer equipped with an Olympus BX41 optical microscope. The

246 instrument employed a 532.21 nm laser wavelength, a 400 μm confocal hole, a grating with 1800

247 grooves/mm, an entrance slit width of 100 μm and 50× long-distance objective lens. The integration

248 time of each scan ranged between 3 and 5 s.

249

250

251 2.7 Stable isotope analysis

252 The stable isotope composition of the modern specimens was analysed on carbonate powder sampled

253 after heating the shells. Carbonate material was obtained from each shell by microdrilling the inner layer

254 (nacre) with a Rexim Minimo dental drill mounted to a stereomicroscope and equipped with a conical

255 drill bit of 300 µm in diameter (Komet/Gebr. Brasseler GmbH & Co. KG, model no. H52 104 003). A

256 Thermo Fisher MAT 253 gas source isotope ratio mass spectrometer in continuous flow mode coupled

257 to a GasBench II at the Institute of Geosciences, University of Mainz was used to analyse the carbonate

258 samples by phosphoric acid digestion (70 °C for 120 min). The isotope data were calibrated against a

259 NBS-19 calibrated Carrara marble standard (δ18O = - 1.91 ‰). The average internal precision (1σ) was

260 0.03‰ and the external reproducibility (1σ) was 0.05‰. The δ18O profiles of the heated shells were

261 compared to the δ18O profiles of the control specimens, which were not exposed to high-temperature

262 treatments.

263

264

265 2.8 Radiocarbon dating

266 For radiocarbon dating, small chips were taken near the apex of three P. turbinatus specimens. In order

267 to avoid any contamination from sedimentary calcium carbonate or a mixture of different shell layers,

268 the external layer of the shells was physically removed. The fragments of nacre were further processed

269 at the Curt-Engelhorn-Center for Archaeometry, Mannheim, Germany. The material was acidified in

270 phosphoric acid at 70 °C by using an Autosampler system and measured using the MICADAS system

271 (Kromer et al., 2013; Wacker et al., 2013). The results were calibrated with the Marine13 curve (Reimer

272 et al., 2013) and corrected using the Mediterranean reservoir correction (ΔR) of 58 ± 85 14C years

273 (Reimer and McCormac, 2002) using OxCal v.4.3.2.

274

275

276 2.9 Statistical analyses

277 ANOVA, Mann-Whitney and Tukey´s HSD post hoc tests were performed using the software PAST to

278 test whether the 5-minute heat exposure had a significant effect on shell geochemistry as well as whether

279 the different durations changed the oxygen isotopic signatures.

280

281

282

283 3. Results

284 3.1 Experimental results on the effect of roasting on the shells

285 At macroscopic level, the 5-minute thermal treatment caused a similar effect as reported for P.

286 turbinatus in the previous experiment by Milano et al. (2016), in which shells were roasted for 20

287 minutes and 60 minutes. Visible alteration of the shell surface occurred at temperatures higher than 200

288 °C. For instance, the typical background cream coloration (Pantone: 7497 C) alternated with pigmented

289 blotches (438 C) started to change at 300 °C, when the shells acquired a dark coloration with a tendency

290 toward a brown shade (background: 7531 C; blotches: 7518 C). At 400 and 500 °C, the shells became

291 grey (background: 404 C; blotches: 417 C) and the outer shell layer started to detach. At 600 °C, the

292 coloration only slightly changed (background: 416 C; blotches: 417 C) and the material brittleness

293 increased, whereas at 700 °C, the coloration turned into a cream-white tone (background and blotches:

294 401 C; Fig. 2A). Furthermore, the heat exposure induced a 14 to 25% weight loss, with a greater loss at

295 higher temperatures (Fig. 2B).

296

297 At the microstructural level, roasting at 200 °C did not cause any visible change of the shell

298 microstructure. First-order and second-order prisms (oSL) and platelets (iSL) of the heated samples did

299 not appreciably differ from those of unheated samples. At 300 °C, the oSL did not show any visible

300 alteration whereas the nacre platelets became slightly porous with nanometer sized holes appearing on

301 their surfaces (Fig. 2C-D). At 400 °C, the single second-order prisms of the oSL were still recognizable.

302 However, sheets of organic material emerged between the first-order prisms partially covering the shell

303 surface. The platelets of the nacre started to fuse and large cracks developed on the surface. After

304 heating at 500 °C, the oSL underwent a significant transformation with the formation of new irregular-

305 shaped units surrounded by organic sheets. Meanwhile, the iSL surface was partially covered by organic

306 matter which emerged from the inter-lamellar voids that formed at 400 °C (Fig. 2C-D). The appearance

307 of the oSL at 600 °C is very similar to the previous treatment (500 °C) with an enhanced presence of

308 organic material. The platelets of the nacre showed a higher degree of fusion in every direction with

309 large quantities of organics in between them. At 700 °C, the surface of the biomineral units of the oSL

310 units became more compact with a decrease in porosity. At the same time, the agglomeration of the iSL

311 platelets led to the formation of new boundaries delineating larger and irregular units (Fig. 2C-D).

312

313 The shell mineralogy was also affected by heat (Fig. 2C-D). Originally, the shells were fully

314 aragonitic as indicated by Raman peaks at 152 cm-1,180 cm-1, 207 cm-1,707 cm-1and 1086 cm-1. The

315 transition from aragonite into calcite started at 400 °C in the oSL and is indicated by an extra peak at

316 281 cm-1 (Fig. 2C). At this temperature, however, the iSL was still completely aragonitic. At 500 °C,

317 both layers were transformed into calcite with peaks at 153 cm-1, 281 cm-1, 713 cm-1 and 1086 cm-1.

318

319 Similar to our previous results (Milano et al. 2016), the shell oxygen isotope composition changed

18 320 in response to the heat treatment. The δ Oshell values of the control sample ranged between - 1.0 and +

321 1.0 ‰. The oxygen isotope range is more negative than the range obtained by the control shells used in

322 Milano et al. (2016). This offset reflects the difference of the local environments. The shells from

323 Portugal used in the present study were collected from an area characterized by much larger freshwater

18 324 input, and hence lower δ Oshell values, than the specimens from Libya used in Milano et al. (2016). The

325 oxygen isotope composition of shells processed at 200 °C and 300 °C for 5 mins ranged between - 0.5

326 and + 0.7 ‰ (Fig. 3A). According to the Mann-Whitney u-test there is no statistical difference between

327 these heated samples and the control shells (p > 0.05; Fig. 3B). The effect of the shell exposure at 400

328 °C could not be tested, because the nacre layer was too thin to obtain sufficient carbonate powder. As

18 329 expected, higher temperatures (500 - 700 °C) had a significant influence on δ Oshell. Average values

330 were offset by -1.5 ‰  0.2 with respect to the average values of the control material. Minimum and

331 maximum values were offset by -0.8 ‰  0.1 and -0.8 ‰  0.7, respectively (Fig. 3A). The oxygen

332 isotopic composition of the shells heated over 500 °C was statistically different from the lower

333 temperature treatments and the control conditions (Mann-Whitney test p < 0.05; Fig. 3B). A comparison

334 between the current data and the results published in Milano et al. (2016) was used to determine the

18 335 influence of the duration of the heating process on the shell. The offsets of the δ Oshell values from the

336 respective control values were calculated for the three duration treatments (5, 20 and 60 min). The

18 337 offsets of minimum, average and maximum δ Oshell values were statistically similar among all treatment

338 durations (ANOVA p > 0.05).

339

340

341 3.3 Archaeological shells: structural and mineralogical organization

342 The P. turbinatus specimens from the Haua Fteah presented inter-individual differences in their

343 appearance at macroscopic scale (Fig. 4). All of the Mesolithic specimens (Trench M) showed a

344 homogenous dark grey surface with areas of partial detachment of the oSL (i.e. M002_1 in Fig. 4D,

345 Supplementary material Fig. S1). The nacre was less uniform and lighter grey in colour. In general,

346 these shells looked similar to those of the experiment roasted at 600 °C (Fig. 2A). Two specimens from

347 the Neolithic context 747 (U747_1 and U747_2) were lighter grey and looked similar to shells heated at

348 500 °C (Supplementary material Fig. S1). The third specimen from this layer (U747_3) showed an

349 iridescent iSL and an overall coloration likely related to lower heating temperatures (i.e., 300 °C;

350 Supplementary material Fig. S1). All four Neolithic P. turbinatus specimens from context 743 were

351 light brown in colour and showed iridescent nacre (i.e. U743_3 in Fig. 4B). The same applied to the two

352 shells from context 742 (U742_2 and U742_4; Fig. 4A, Supplementary material Fig. S1), whereas

353 U742_1 and U742_3 were grey as if subjected to higher temperatures (Fig. 4C, Supplementary material

354 Fig. S1).

355

356 In good agreement with the observations of the macro appearance of the shells, microstructures

357 varied greatly between individuals. The prisms in the oSLs of the Mesolithic specimens from were

358 replaced by porous irregular units and the nacre platelets were fused together into larger agglomerates

359 (Fig. 4D, Supplementary material Fig. S2). The integral microstructural reorganization and the

360 subsequent absence of any of original secondary prisms (oSL) and platelets (iSL) recall the structural

361 arrangement of the modern samples exposed to high temperatures (600 °C; Tab. 2). A similar

362 architecture was observed in the Neolithic specimens U747_1, U747_2 and U742_3. On the other hand,

363 all shells from context 743 and specimens U747_3 and U742_4 showed well-defined prismatic

364 structures in the oSL and slightly porous platelets in the iSL, which looked similar to the experimental

365 shells heated at 300 °C (Fig. 4B, Supplementary material Fig. S2). Shell U742_1 represented the only

366 case identifiable as processed at a temperature of ca. 500 °C (Table 2). The prisms of the oSL appeared

367 to have undergone major fusion at 400 °C (Fig. 4C). However, the single elongated prisms were still

368 partially visible and they were not completely transformed as in the case of the material burned at 600

369 °C. A similar pattern was observed in the nacre, where the agglomerations of platelets started to define

370 new structural units (Fig. 4C). The microstructures of specimen U742_2 were perfectly preserved and

371 did not show any sign of thermal alteration (Fig. 4A, Table 2).

372

373 According to Raman spectroscopy, seven shells were entirely aragonitic and eleven were calcitic

374 (Fig. 5). All Mesolithic specimens and four of the Neolithic specimens consisted of calcite (Fig. 5A),

375 whereas the remaining specimens from Trench U were aragonitic (Fig. 5B). No transition phase was

376 detected and the two shell layers always shared the same mineralogy. Based on the results presented

377 above, the calcitic shells were categorized as having been exposed to temperatures in excess of 500 °C

378 (Tab. 2).

379

380 Similar to the P. turbinatus, some archaeological specimens of A. uropigimelana and T. palustris

381 from the United Arab Emirates sites displayed an altered overall appearance (Fig. 6). Shells K4 SL

382 Ana3, KSM UBL Ana2, KK1 Ana3 and KK1 Ana4 showed the characteristic homogenous creamy

383 colour of unaltered A. uropigimelana (Fig. 6A; Supplementary material Fig. S3). A slightly darker

384 shade, especially on the inner shell surface, was visible in KK1 Ana4. Two shells, KS Ana1 and K4 SL

385 GA1, showed a grey shell surface. The first was characterized by a light tone, whereas the latter was

386 darker with shaded areas on the inner shell surface clearly indicating heat exposure (Fig. 6A;

387 Supplementary material Fig. S3). In the case of T. palustris, one specimen (K4 BZ UGT 1) had its

388 surface coloration preserved, whereas a second specimen (K4 BZ GT 1) was very dark (Fig. 6B).

389

390 The well-preserved A. uropigimelana specimens K4 SL Ana3, KSM UBL Ana2, KS Ana1, KK1

391 Ana3 and KK1 Ana4 showed well-defined crossed-lamellar structures in the oSL, linear-crossed

392 lamellae in the mSL and irregular complex-crossed lamellae in the iSL. The excellent preservation state

393 was also corroborated by the presence of organic microtubules across the shell layers (Fig. 6A;

394 Supplementary material Fig. S3). Conversely, shell K4 SL GA1 displayed altered microstructures. The

395 first order lamellae of the oSL and mSL were still visible, whereas the morphometric characteristics of

396 the third order lamellae were changed. The units were fused together in large agglomerates with new

397 boundaries (Fig. 6A). These features recall the oSL of Phorcus spp. heated at 600 °C. In the iSL, the

398 lamellae lost their individuality and elongated shape, forming compact clusters of material separated by

399 organic sheets and cracks (Fig. 6A). The T. palustris specimen K4 BZ UGT 1 showed the typical simple

400 and linear-crossed lamellar microstructures (Fig. 6B), whereas in specimen K4 BZ GT 1 lamellar

401 biomineral units were transformed into large irregular-shaped assemblages in the oSL and iSL. In the

402 mSL, small granular carbonate units with well-defined boundaries prevailed (Fig. 6B). Such alteration

403 was likely related to high temperatures (≥ 500 °C). However, in this case an accurate estimate of the

404 processing temperature cannot be provided based on the microstructures, because no such architecture

405 was previously observed in the mSL.

406

407 Typical aragonite Raman spectra occurred in all but two A. uropigimelana and T. palustris

408 specimens. The mineralogy of K4 SL GA1 and K4 BZ GT 1 was entirely calcitic (Fig. 7A-B).

409

410

411 3.4 Radiocarbon dating of P. turbinatus from Haua Fteah

412 Specimen U742_2, with no sign of heat exposure (Fig. 4A), was dated to 7184 ± 21 14C yr BP calibrated

413 to 7772 - 7438 cal. BP (2σ). Specimen U742_1, with signs of high temperature exposure (Fig. 4C), was

414 dated to 7300 ± 21 14C yr BP calibrated to 7901 - 7556 cal. BP. both these dates fit with existing 14C

415 dates for context U742. Specimen U742_3 (Fig. 4B), with burning marks, was dated to 6378 ± 21 14C yr

416 BP calibrated to 7002 - 6558 cal. BP. These dates fit with the current chronostratraphic understanding of

417 Context U743, which is paired with U742 stratigraphically (Fig. 8). Associated 14C radiocarbon ages are

418 available from other materials dated in Trench U are also included in this study for comparative

419 purposes (Table 3). The relatively broad spread of dates from U742 fits sedimentological model for this

420 part of the Haua Fteah deposition sequence and all sit within the existing age range for these context.

421 Context U742 is one of a group of chronologically identical contexts (with U743, U744, U746 and

422 U747) that form a cross section of a debris flow that contains archaeological material that can be

423 attributed to the Neolithic.

424

425

426

427 4. Discussion

428 4.1 A comparative approach to thermal-induced diagenesis in biogenic carbonates

429 Diagenesis encompasses all physical and chemical processes occurring to the tissues after the death of

430 the animal. The present study focuses exclusively on thermally-induced diagenesis associated with heat

431 exposure. High temperatures accelerate a process that naturally would require thousands of years. When

432 dealing with archaeological remains it is important to differentiate between the two types of diagenetic

433 processes to ensure a correct interpretation of the results. In order to do so, the whole shell assemblage

434 needs to be taken into consideration. Specimens from a certain archaeological layer are subjected to

435 similar environmental conditions after burial (i.e. depth, sediment composition and soil pH). Therefore

436 they are likely to share similar preservation conditions. The overall preservation state indicates whether

437 natural diagenesis altered the shells or not. Any deviation from the average preservation state is likely

438 related to different processes. In our case, shells without evidence of burning are generally well-

439 preserved suggesting that natural diagenesis did not occur (Prendergast et al., 2016). Any altered shell is

440 likely to be related to heat-induced diagenesis. Although this approach allows a reasonable distinction

441 between natural and artificial diagenesis, further studies on naturally-altered shell material are needed to

442 characterize the structural and mineralogical key features relative to this type of diagenesis.

443

444 As previously demonstrated by Milano et al. (2016), the macroscopic and microscopic appearance

445 of Phorcus spp. shells gradually altered as the processing temperatures increased. Although the thermal

446 exposure time was reduced to 5 minutes in the present study, similar mineralogical and geochemical

447 changes were observed. The only exception is the temperature at which the aragonite-to-calcite

448 transformation of the oSL occurred, i.e., 400 °C in the present study and 300 °C in Milano et al. (2016).

449 Based on this discrepancy, we suggest that the duration of the thermal exposure may play a role in the

450 development rate of such taphonomic processes. However, time is only of secondary importance in

451 comparison to the actual processing temperature. In good agreement with the most of our results,

452 Aldeais et al. (2016) demonstrated that the mineralogy of Cerastoderma edule and Scrobicularia plana

453 shells showed no differences after roasting for 5 and 20 minutes. In abiogenic and biogenic minerals, the

454 aragonite-to-calcite and microstructural transformations have been previously observed to occur very

455 rapidly at critical temperatures around 400 °C and 500°C, respectively (Lécuyer, 1996; Koga et al.,

456 2013). As a consequence, it will most likely be impossible to reconstruct the processing duration based

457 on the analysis of archaeological shells, but it is possible to reconstruct the temperatures at which they

458 were exposed. Consequently, it may be possible to postulate the type of fire or fuel that created those

459 temperatures.

460

461 P. lineatus shells lost weight after roasting, especially at temperatures above 300 °C (21.5 %

462 weight loss). Heat-induced weight losses have previously been reported from biogenic and abiogenic

463 minerals (Yoshioka and Kitano, 1985; Balmain et al., 1999; Bourrat et al., 2007). Thermal exposure

464 induces a three-step release of water and organic content. At first, the endothermic reaction provokes

465 dehydration through evaporation of the water incorporated in the mineral phase (Yoshioka and Kitano,

466 1985; Perić et al., 1996; Huang et al., 2009). Then, at temperatures between 250 °C and 450 °C, the

467 organic matter in the shell starts to degrade (Bourrat et al., 2007; Huang et al., 2009). In Phorcus spp.,

468 this temperature range conforms to the appearance of large organic sheets partially masking the

469 carbonate microstructures (Fig. 2). The degradation intensifies at higher temperatures (> 500 °C) and is

470 associated with weight losses up to 50 % (Balmain et al., 1999; Zolotoyabko and Pokroy, 2007; Huang

471 et al., 2009; Villagran et al., 2011).

472

473 In the present study, temperature was increased stepwise by 100 °C steps to systematically

474 investigate structural and mineralogical changes of the shells. The outer shell layer of P. lineatus started

475 to transform from aragonite to calcite at 400 °C. This process was completed at 500 °C. In the case of

476 the nacre, no such broad transition phase was detected. Conversion to calcite took place at 500 °C, at

477 which point the iSL was entirely calcitic. According to previous studies, however, polymorphic

478 transition of mollusk shells already started at temperatures of 250 - 350 °C, and conversion to calcite

479 was completed between 400 °C and 500 °C (Lécuyer, 1996; Maritan et al., 2007; Aldeias et al., 2016).

480 Conversion temperatures seem to vary between species. As shown by Aldeias et al. (2016), the

481 mineralogy of C. edule and S. plana reacts differently to heating. In C. edule polymorphic conversion

482 occurs between 250 and 500 °C, whereas in S. plana the transformation only starts at 350 °C and is

483 completed at 400 °C. The observed differences in polymorphic conversion temperatures may be

484 explained by specific structural properties of the materials. Crystal morphology and crystallography

485 were previously identified as potential factors controlling mineralogical transformations (Koga et al.,

486 2013). In abiotic systems, aragonite crystals split and released water trapped within them. Furthermore,

487 calcite nucleation occurs preferably along mineral cracks and twin boundaries (Bischoff and Fyfe, 1968;

488 Liu and Yund, 1993). For these reasons, crystal characteristics and positions of boundaries influence the

489 conversion reaction (Koga et al., 2013). The architectural diversity among mollusk species and shell

490 layers may be the origin of the variable reaction of different biominerals to thermal stress (Kobayashi

491 and Samata, 2006). Aragonitic mineral units tend to lose density and acquire nanopores on their

492 surfaces, while a network of cracks forms in the material (Perdikouri et al., 2011; Villagran et al., 2011;

493 Gomez-Villalba et al., 2012). These features were also observed in the present study in P. lineatus

494 specimens roasted at 400 °C, i.e., before full conversion into calcite.

495

496 Given the complexity in the thermal behaviour of biogenic materials, the three-pronged approach

497 of mineralogical, macro- and microscopic observations is essential to better understand the thermally-

498 induced diagenetic processes. The identification of specific changes in the shell can be used as indicator

499 of shell processing temperatures, although possibly with some differences from species to species.

500

501

502 4.2 Implications of heat exposure for palaeoenvironmental reconstructions

503 Heat exposure can induce an exchange of oxygen isotopes with the surrounding environment, drastically

504 altering initial shell isotopic composition (Andrus and Crowe, 2002; Milano et al., 2016). Temperatures

18 505 higher than 300 °C are observed to cause a significant decrease in δ Oshell (Milano et al., 2016). Our

506 results are in good agreement with the previous findings suggesting that the use of burnt mollusk shells

18 507 may have important implications for palaeoenvironmental reconstructions. A decrease in δ Oshell

508 translates into an overestimation of the reconstructed water temperature, which can jeopardize the

509 reliability of related reconstructions. Furthermore, burning at high temperatures can influence the results

510 of palaeoseasonality studies by incorrectly estimating the season(s) of mollusk collection, which in turn

511 may affect the interpretation of hunter-gatherer mobility and settlement use.

512

513 It is recommended to discard shells with signs of thermal exposure for isotopic analyses. However,

514 it has to be noted that not all heating intensities imply isotopic alteration. Low temperatures (< 300 °C)

18 515 do not induce significant changes in the δ Oshell. Shells exposed to such temperatures can still be used to

516 accurately estimate palaeotemperatures. However, a careful analysis is needed to differentiate between

517 the different heat exposures and to ensure an optimal sample selection for isotopic studies. It is also

518 advisable to avoid any form of pre-treatment involving heating, which was commonly adopted in the

519 past as method for organic matter removal (Mook, 1971).

520

521

522 4.3 Prehistoric fire events and shell middens

523 Different types of fire events can be associated to shell middens, according to their nature and use. The

524 first evidence for controlled fire is dated to around 300,000-250,000 years ago and it is related to a

525 change in hominin habits, especially concerning diet (James et al., 1989; Shahack-Gross et al., 2014;

526 Gowlett, 2016). Among the main benefits of using fire for dietary purposes are the maximization of

527 energy gain and food digestibility and the development of social abilities such as storing, sharing and

528 processing food (Ragir, 2000; Wrangham and Conklin-Brittain, 2003; Wrangham, 2009; Wrangham and

529 Carmody, 2010). Several ethnographic studies suggest that cooking may play an important role in

530 shellfish consumption (Percy, 1608; Meehan, 1977, 1982). Cooking is generally known to facilitate the

531 extraction of the edible portion. In fact, the heat weakens the muscle attachment of the mollusks causing

532 the valves to open (bivalves) and the flesh to detach more easily. In Australia and North America

533 mollusks were placed over or underneath fire, coals or hot stones (Kroeber and Barrett, 1960;

534 Beaglehole, 1974; Bailey, 1977). In other cases, they were cooked in earth ovens (Meehan, 1982;

535 Waselkov, 1987; Thoms, 2008). Additionally, steaming was adopted as an alternative technique by

536 some ethnic groups such as the Maori in New Zealand and the Quileute and the Tlingit in the Pacific

537 Northwest (Best, 1924; Reagan, 1934; Newton and Moss, 1984). Generally, the cooking processes were

538 rather fast, possibly lasting only a few minutes (Meehan, 1977; Thoms, 2008). After being cooked,

539 mollusk soft tissues were sometimes dried or smoked to enable storage for longer periods of time and

540 for trading purposes (Greengo, 1952; Moss, 1993). In addition to food preparation, fire could have being

541 used for trash disposal (Ford, 1989). Ethnographic observations by Meehan (1982) describe occasional

542 burning of food debris by igniting grass on top of large mounds of disposed shells. However, in other

543 cases, the shells were arranged in heaps and covered with sand, without involving any fire (Bird and

544 Bird, 1997). Alternatively, Mougne et al. (2014) suggested the shell refuses to be directly thrown into a

545 fire.

546 To contextualize our results in the light of the ethnographic records, the experimental work by

547 Aldeias et al. (2016) simulating different firing techniques offers a valuable source of information. The

548 authors showed that when a fire was set on top or underneath a pile of shells, there was a great degree of

549 heterogeneity in the shell mineralogical response. The heat diffusion affected differently the single

550 specimens according to their position in respect to the heat source, resulting in a mixture of aragonitic

551 and calcitic shells. Our results indicate that the distribution of specific burning-related features is

552 generally shared among the specimens of the same layer suggesting a homogenous exposure to heat. An

553 exception to this trend is represented by the shells from the Neolithic layer U742, which will be

554 discussed further below. Such uniform heat exposure likely implies heating of relatively small quantities

555 of shells with a similar position in respect to the fire whereas large amounts of material, characteristic of

556 disposal mounds, would induce a significant spatial heat gradient and diverse thermal response.

557 Furthermore, concerning the food disposal by throwing the shells into the fire, this involves direct

558 contact with extreme temperatures. Under these circumstances, the shells would become extremely

559 brittle and prone to a rapid disintegration with limited chances of fossil ‘survival’ (Milano et al., 2016).

560 In this perspective, our data conform better with firing events associated to food processing rather than

561 food disposal. However, further analysis on additional material is needed before unequivocally

562 discriminate between the two different types of fire.

563

564 Estimation of shellfish heating temperatures from the Haua Fteah suggests a clear distinction

565 between the two archaeological contexts studied here. The seven Mesolithic shells all appear to have

566 been exposed to temperatures between 600 and 700 °C. These temperatures can be reached when the

567 shellfish are situated in close proximity to the heat source without being directly in contact with it.

568 Temperatures between 633 and 781 °C were recorded when experimentally roasting the mollusks on a

569 surface with fuel and fire on top of them (Aldeias et al., 2016). Such temperatures fit well with our

570 observations on the shell remains. However, it has to be considered that roasting temperatures also

571 largely depend on the type of fuel used. In the experiments by Aldeias et al. (2016), pine wood and pine

572 needles were adopted, whereas grass material is known to burn at lower temperatures (max. 430 °C;

573 Wolf et al., 2013). In the light of these observations and our own results, shellfish during the Mesolithic

574 phase at the Haua Fteah were probably processed underneath woody fires.

575

576 In contrast, the Neolithic shells from the Haua Fteah layers U747 and U743 have features pointing

577 to lower temperature exposure: ca 71 % were heated at temperatures around 300 °C. The drastic increase

578 in remains exposed to lower temperatures in the Neolithic phase can be explained by changes in the

579 shellfish processing. A potential interpretation is related to the appearance of the first pottery during the

580 Neolithic, which could have been used to prepare shellfish (McBurney, 1967; Douka et al., 2014;

581 Prendergast et al., 2016). Although the use of vessels for processing marine resources has not been yet

582 attested around the Mediterranean during the Neolithic (Debono Spiteri and Craig, 2014), it has been

583 previously observed in other regions (Craig et al., 2011; Taché and Craig, 2015). An experiment

584 conducted by Maggetti et al. (2011) showed that clay pots exposed to fire can reach temperatures of

585 around 300 °C after ca. 10 minutes, supporting our observations in the shells of layers U747 and U743.

586 A second interpretation of the lower temperature exposure during the Neolithic encompasses similar

587 heating processes with the adoption of substantially different types of fuel. As mentioned above, the

588 usage of grasses as fuel could explain the lower burning temperatures experienced by the Neolithic

589 shells.

590

591 As an exception to the general trend observed in the results, the shells from unit U742 present

592 heterogeneous burning features related to temperatures from 300 °C to 600 °C, with one shell without

593 any burning sign. On the basis of our observations two hypotheses can be discussed. Different burning

594 degrees may indicate the presence of a thermal gradient related with the use of fire for disposal

595 purposes. Alternatively, the shell may reveal changes in cooking processes in relation to the type of fuel

596 used and/or cooking vessels adoption.

597

598 In the case of the archaeological samples from the United Arab Emirates, both species showed

599 evidences related to heating at 500 - 600 °C. Archaeological sites in this area dated to the late Bronze

600 Age or Classic Wadi Suq period (2000 - 1500 BC) are characterized by a large amount of pottery

601 remains such as beakers and spouted globular jars (Carter, 1997). In this region, food processing with

602 pots seems to have appeared during the Bronze Age, whereas roasting on fire was the preferred method

603 in earlier times (Händel, 2013). However, the small sample size considered in the present study does not

604 allow us a more precise interpretation. In order to better understand the evolution of shellfish processing

605 in the area, additional specimens are needed.

606

607 Together with intentional fires, natural burning events can occur in shell middens. A distinction

608 between intentional and unintentional firing events can hardly be determined on the basis of shell

609 thermal response. However, as in the case of disposal by directly throwing the shells into fire, long-

610 lasting uncontrolled fires would lead to the complete disappearance of the shell remains within the

611 midden. In our case, the preservation of large fragments and whole shells suggests that the heat exposure

612 was likely not related to long-lasting natural fires. However, rapid fires cannot fully be discharge as

613 potential burning sources.

614

615

616 4.4 Heat exposure and radiocarbon dating robustness

14 617 Previous studies on shell-tempered pottery showed that these materials provide consistent CAMS

618 dates (Rick and Lowery, 2013). To obtain resistant ceramics, shell-tempered pottery is typically fired at

619 300 - 600 °C and - after cooling - fired again to 500 - 600 °C (Kingery et al., 1976; Herbert, 2008). Such

620 temperatures do not seem to affect radiocarbon ages on the shell inclusions, which fit well with dates

621 from charcoal and bone fragments of the same context (Rick and Lowery, 2013). Shells analysed in the

622 present study did not differ from already available ages of the same stratigraphic context. According to

623 the structural and mineralogical analyses, two of the samples were burned at 500 °C and 600 °C,

624 whereas the third one was not burned. The relatively homogeneous AMS dates suggest that heating does

625 not affect radiocarbon ages, even when polymorphic transformation has occurred. This may be due to

626 the fact that the extremely small quantity of carbon remaining in the material (ca 0.1 wt%) is still

627 enough to allow AMS dating (Lanting et al., 2001). Furthermore, physical alterations of the biomineral

628 such as structural unit enlargement and recrystallization seem to provide a barrier protecting the

629 remaining carbon (van Strydonck et al., 2005).

630

631

632 4.5 Methods to identify thermal exposure on shell remains

633 Our results show that the overall appearance of shells alone is an insufficient approach to identify

634 heating-related changes (Milano et al., 2016). Although useful in cases of exposure to high temperatures

635 with evident burning marks, examination by the naked eye has important limitations when it comes to

636 lower temperatures. For instance, based on the outer shell layer, specimen U742_2 seemed to have been

637 exposed to 300 °C. However, according to more detailed analysis using SEM it turned out to be

638 untreated.

639 The analysis of the mineralogical response to heat is valuable when combined with other methods.

640 However, it may prove ambiguous when used alone. The archaeological material analyzed for this study

641 did not show any carbonate polymorph transition phase. Therefore, Raman spectroscopy could only help

642 to distinguish between temperatures below and above 500 °C. Our current and previous results indicate

643 that a combined approach of optical microscopy, SEM and Raman spectroscopy is the most suitable

644 solution to identify alterations related to shellfish processing. The combined use to these techniques will

645 provide more robust data on heating exposure, whether the aim of the research is to preclude heated

646 shells for palaeotemperature reconstructions (Milano et al., 2016) or to examine the food processing in

647 more detail. Although the experimental phase was specifically designed for Phorcus spp., the

648 observations showed similarities with the thermal behaviour of A. uropigimelana and T. palustris. Such

649 analogy in the responses to heat suggests that temperature estimation can be achieved among different

650 mollusk species even without a specific foregoing experimental phase. Results of our findings can be

651 used to answer a broad spectrum of different questions in the framework of shell midden research and

652 shell-tempered pottery studies. Further studies on calcite mollusk shells are needed to complement the

653 existing data on aragonite shells. This will improve the understanding of the biomineralized tissue

654 thermal response in the case of a more stable calcium carbonate phase.

655 Although the methodology developed offers a powerful toolbox to reconstruct shell thermal

656 exposure, care must be paid when interpreting the results in the archaeological context. As discussed, the

657 preservation conditions of the shells (i.e., entire shells/small fragments; homogenous/heterogeneous

658 burning features) can offer insights into the type of fire event. However, these parameters cannot resolve

659 with certainty the exact nature of the fire (intentional/natural; cooking-related/disposal-related).

660 Furthermore, it is important to mention that, given the irreversible nature of the structural and

661 mineralogical thermal response, multiple burning events cannot be discerned. As for any study on burnt

662 remains, the signature preserved is related to the highest temperature experienced by the shell material.

663

664

665

666 5. Conclusions

667 The experimental component of this study has confirmed that shellfish processing methods

668 encompassing temperatures greater than or equal to 300 °C do significantly alter mollusk shell material,

669 but that the duration of the heating does not appear to significantly affect the behaviour of the shell to

670 thermal treatment. According to these findings, structural and chemical changes occur as soon as the

671 shell is exposed to high temperatures. The results also confirm that using burnt shells for

672 palaeoenvironmental reconstructions can produce incorrect temperature estimations and affect the

673 interpretation of environmental and human-related conditions. One proviso to these findings is that the

674 experiments were conducted on fresh shell from which the flesh had been removed when the live

675 mollusks were collected, and further experimental work is needed to establish what impact the presence

676 of body fluids might have on the results. However, burned shells remain a valuable source of

677 information. The experimental changes observed in the modern shells at macro-, microstructural and

678 mineralogical scales have allowed us to determine a set of criteria to detect heating temperatures that can

679 easily be applied to archaeological shells. Analysis of archaeological shells of P. turbinatus from the

680 Haua Fteah cave in coastal northeast Libya revealed that during the Mesolithic the shells were uniformly

681 exposed to high temperatures (shells in close proximity to the heat source) whereas during the Neolithic

682 lower temperatures were recorded. Although, the use of fire for non-dietary purposes cannot be

683 completely ruled out, the trend in our results can be interpreted as a temporal change in shellfish

684 preparation.

685 Analysis of archaeological shells of A. uropigimelana and T.palustris from Neolithic and Bronze

686 Age shell midden sites on the coast of the United Arab Emirates found that some specimens did not

687 show any evidence of burning while others showed indications of high-temperature exposure. Although

688 burning affected the shell structure and mineralogy, it does not appear to have a significant influence on

689 radiocarbon ages suggesting the possibility of using burnt remains for dating purposes. The fact that the

690 thermal reaction of different species is similar provides confidence in applying the techniques outlined

691 in this study at an interspecific level.

692

693

694

695 Acknowledgements

696 The authors thank Michael Maus and Dr. Tobias Häger for helping during isotope analysis and Raman

697 measurements and Dr. Lucy Farr for selecting the shells from the Haua Fteah assemblage. Funding for

698 this study was kindly provided by the EU within the framework (FP7) of the Marie Curie International

699 Training Network ARAMACC (604802) to SM and by the Alexander von Humboldt Postdoctoral

700 Fellowship (1151310) and McKenzie Postdoctoral Fellowship to AP. The Haua Fteah excavations were

701 undertaken with the permission of the Libyan Department of Antiquities and with funding to GB from

702 the Society for Libyan Studies and from the European Research Council (Advanced Investigator Grant

703 230421), whose support is also gratefully acknowledged. The three anonymous reviewers are thanked

704 for their constructive comments on the manuscript.

705

706

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1022

1023

1024

1025 Figure and tables

1026

1027 Fig. 1. Map showing the localities encompassed in the study. Red star: Praia do Tamariz, Portugal,

1028 where the modern Phorcus lineatus were collected. Grey star: Haua Fteah cave, Libya, where the

1029 archaeological Phorcus turbinatus were excavated. Blue star: sites in the UEA from which the

1030 archaeological remains of Anadara uropigimelana and Terebralia palustris were excavated.

1031

1032 Fig. 2. Effects of the 5-minute heating experiment on P. lineatus shell. (A) Overall shell appearance

1033 after the thermal exposure at different temperatures. Scale bars = 5 mm. (B) Shell weight loss in

1034 response to heat. (C) Effect of heat on the outer shell layer (OSL) mineralogy and microstructures. The

1035 black arrow shows the extra peak in the 400 °C Raman spectrum indicating the transition phase from

1036 aragonite into calcite. (D) Thermal response of the inner shell layer (ISL) mineralogy and

1037 microstructures.

1038

1039 Fig. 3. Thermal behaviour of shell oxygen isotopes. (A) Oxygen isotope composition after roasting at

18 18 1040 the different temperatures. Gray squares=maximum δ Oshell; open triangles=average δ Oshell; black

18 1041 circles=minimum δ Oshell. (B) The table refers to the different cooking durations tested in the present

1042 study and the previous study by Milano et al. (2016). Green cells indicate statistical similarity between

1043 isotope values of the heated and control shells (p > 0.05). Red cells indicate statistical difference (p <

1044 0.05). The yellow cell relates to a disagreement in the case of the specimen roasted at 300 °C for 20

1045 minutes. Its isotopic signature was statistically similar to the one of the control shell but different to the

1046 second control sample.

1047

1048 Fig. 4. Macroscopic appearance and microstructural organization of P. turbinatus shells from (A-C)

1049 Neolithic contexts and (D) Mesolithic contexts in the Haua Fteah cave. The first row of SEM images

1050 refers to the prismatic microstructures of the OSL. The second row displays the nacre platelets of the

1051 ISL. The visible changes in structural organization denote possible exposure to heating processes. Scale

1052 bars if not otherwise indicated = 5 mm.

1053

1054 Fig. 5. Raman spectra of P. turbinatus ISLs from (A) Neolithic contexts and (B) Mesolithic contexts in

1055 the Haua Fteah cave. All Mesolithic specimens and four Neolithic specimens were calcitic (black lines);

1056 the rest of the Neolithic specimens were aragonitic (grey lines).

1057

1058 Fig. 6. Macroscopic appearance and microstructural organization of the archaeological shells from the

1059 United Arab Emirates. (A) Whole shell and SEM images of two A. uropigimelana specimens. Shell

1060 KSM UBL Ana2 showed the typical microstructural architecture of the species. Highlights on the SEM

1061 images indicate the first and third order units of the crossed-lamellar structures as well as the organic

1062 microtubules perforating the shell material. Specimen K4 SL GA 1 shows the alteration of the overall

1063 color and microstructures, possible related to heat treatment. (B) Whole shell and SEM images of two T.

1064 palustris specimens with regular (K4 BZ UGT 1) and altered microstructures and appearance (K4 BZ

1065 UGT 1). Scale bars if not otherwise indicated = 1 cm.

1066

1067 Fig. 7. Raman spectra of (A) A. uropigimelana and (B) T. palustris from the United Arab Emirates

1068 archaeological sites. Specimens K4 SL Ana2 and K4 BZ UGT 1 preserve their aragonitic OSL, MSL

1069 and ISL (grey lines). Specimens K4 SL GA 1 and K4 BZ GT 1 display calcite in all shell layers (black

1070 lines).

1071

1072 Fig. 8. Distribution of calibrated 14C dates of Haua Fteah Trench U. Color-coding corresponds to the

1073 different materials used in the dating. Green = H. Melanostoma shells; blue = well-preserved P.

1074 turbinatus; grey = P. turbinatus from this study; black = charcoal.

1075

1076

1077

1078 Table 1. List of studied specimens and details on their provenance. ID Species Provenance Period A001 P. lineatus Portugal Modern A001_M P. lineatus Portugal Modern A001_MR P. lineatus Portugal Modern A200 P. lineatus Portugal Modern A200_M P. lineatus Portugal Modern A300 P. lineatus Portugal Modern A300_M P. lineatus Portugal Modern A400 P. lineatus Portugal Modern A400_M P. lineatus Portugal Modern A500 P. lineatus Portugal Modern A500_M P. lineatus Portugal Modern A600 P. lineatus Portugal Modern A600_M P. lineatus Portugal Modern A700 P. lineatus Portugal Modern A700_M P. lineatus Portugal Modern Haua Fteah, M006_1 P. turbinatus Capsian Libya

Haua Fteah, M006_2 P. turbinatus Capsian Libya Haua Fteah, M005_1 P. turbinatus Capsian Libya Haua Fteah, M005_2 P. turbinatus Capsian Libya Haua Fteah, M002_1 P. turbinatus Capsian Libya Haua Fteah, M002_2 P. turbinatus Capsian Libya Haua Fteah, M002_3 P. turbinatus Capsian Libya Haua Fteah, U747_1 P. turbinatus Neolithic Libya Haua Fteah, U747_2 P. turbinatus Neolithic Libya Haua Fteah, U747_3 P. turbinatus Neolithic Libya Haua Fteah, U743_1 P. turbinatus Neolithic Libya Haua Fteah, U743_2 P. turbinatus Neolithic Libya Haua Fteah, U743_3 P. turbinatus Neolithic Libya Haua Fteah, U743_4 P. turbinatus Neolithic Libya Haua Fteah, U742_1 P. turbinatus Neolithic Libya Haua Fteah, U742_2 P. turbinatus Neolithic Libya Haua Fteah, U742_3 P. turbinatus Neolithic Libya Haua Fteah, U742_4 P. turbinatus Neolithic Libya A. K4 SL Ana3 Oman Sea Bronze Age uropigimelana A. K4 SL GA1 Oman Sea Bronze Age uropigimelana K4 BZ GT1 T. palustris Oman Sea Bronze Age K4 BZ UGT1 T. palustris Oman Sea Bronze Age A. KSM UBL Ana2 Oman Sea Neolithic uropigimelana A. KS Ana 1 Oman Sea Neolithic uropigimelana A. KK1 Ana3 Oman Sea Neolithic uropigimelana

A. KK1 Ana4 Oman Sea Neolithic uropigimelana 1079 1080 1081 1082 Table 2. Reconstruction of shell exposure temperature by using shell overall appearance, 1083 microstructures and mineralogy as proxies in archeological specimens of P. turbinatus. Shell ID Reconstructed cooking temperature Macroscale appearance Microstructures Mineralogy

M006_1 600 °C 600 °C ≥ 500 °C M006_2 600 °C 600 °C ≥ 500 °C M005_1 600 °C 600 °C ≥ 500 °C M005_2 600 °C 600 °C ≥ 500 °C M002_1 600 °C 700 °C ≥ 500 °C M002_2 600 °C 600 °C ≥ 500 °C M002_3 600 °C 700 °C ≥ 500 °C U747_1 500 °C 600 °C ≥ 500 °C U747_2 500 °C 600 °C ≥ 500 °C U747_3 500 °C 300 °C < 500 °C U743_1 300 °C 300 °C < 500 °C U743_2 300 °C 300 °C < 500 °C U743_3 300 °C 300 °C < 500 °C U743_4 300 °C 300 °C < 500 °C U742_1 400 °C 500 °C ≥ 500 °C U742_2 Not cooked Not cooked < 500 °C U742_3 500 °C 600 °C ≥ 500 °C U742_4 500 °C 300 °C < 500 °C 1084 1085

1086 Table 3. Trench U (Haua Fteah) 14C dates. Symbol *1 indicates dates corrected using reservoir offset of 58 ± 85 1087 14C years (Reimer and McCormac, 2002). Symbol *2 indicates dates corrected using reservoir offset of 476 ± 48 1088 14C years (Hill et al., 2017). 1089

Age (cal. BP) Age (cal. BP) Sample ID Lab Code Material Age (14C yr) ± (unmodelled) (modelled) MAMS 30538 U742_1 P. Turbinatus 7300 21 7901 - 7556*1 7869 - 7560*1 U742-1 MAMS 30538 U742_2 P. Turbinatus 7184 21 7772 - 7438*1 7747 - 7461*1 U742-2 MAMS 30538 U742_3 P. Turbinatus 6378 21 7002 - 6558*1 6978 - 6610*1 U742-3 ARO747UG OxA-27391 P. Turbinatus 8171 37 8891 - 8381*1 8776 - 8385*1 ARO747UC OxA-27490 P. Turbinatus 7447 35 8024 - 7652*1 8008 - 7675*1 742 UBA-28873 Charcoal 6965 39 7551 - 7405 7551 - 7405 <5982>/118C 743<5984>/119C UBA-28874 Charcoal 6291 50 6883 - 6627 6880 - 6630 746 <88> A UBA-25915 H. Melanostoma 8670 39 8930 - 8534*2 8840 - 8531*2 743 <83> UBA-25912 H. Melanostoma 7732 37 7850 - 7590*2 7840 - 7605*2 1090

1091 Supplementary material

1092 1093 1094 Fig. S1. Overall appearance of all P. turbinatus excavated from Haua Fteah cave and used in the present study. 1095 The upper seven specimens were excavated from Trench M (Mesolithic). The rest of the specimens were 1096 excavated from Trench U (Neolithic). 1097

1098 60

1099 1100 Fig. S2.SEM images of the microstructural organization of P. turbinatus from Haua Fteah. Trench U = Neolithic 1101 context. Trench M = Mesolithic context. 1102

1103 1104 1105 Fig. S3. Overall appearance and microstructural organization of A. uropigimelana from Southeast Arabia. 1106 1107